Why my Spitz barks only at my immediate neighbour?

[ViosAngel]

My little boy is basically a very friendly and timid puppy (coming to 10mths now). Even his groomer and vet said that he is vey friendly and good natured for a Spitz. Everywhere I bring him, he will wanna make friend with even the passer-bys and usually gets friendly acknowledgement in return to which he sometimes respond with shyness and run behind me to hide.

The thing I don't understand is why he barks very fiercly at my one and only neighbour. The moment he hears them at the door or hear their children making noise, he will sprint to a point nearest to the main door and growl. My doors are always closed.

Some friends told me that maybe its due to the neighbour's kid slamming their door at his face when we were waiting for the lift to go out. But can this be true? Can my otherwise overly friendly boy bear such dislike for just one family? Even the men who came to service the aircon do not receive such treatment from him.

Can someone enlighten me and tell me what I can do to make him less hostile to my neighbour? I don't want my little boy to be branded a rouge dog.